Electrical signals emanating from apparatus on board a vehicle's wheel usually include a significant amount of signal noise, especially when the vehicle is travelling over rough ground. In some cases, the signal component that it is desired to detect is dependent on wheel speed and this complicates the extraction of the desired signal component from the noise.
By way of example, U.S. Pat. No. 7,367,227 discloses a Tire Pressure Monitoring System (TPMS) in which a pair of shock sensors are included in a wheel-mounted tire pressure measuring device, the shock sensors producing electrical signals from which the respective wheel's location on a vehicle can be deduced. The electrical signals produced by the shock sensors include a main signal component, representing the output of the shock sensor, and signal noise. The frequency and amplitude of the signal noise are sufficiently close to the frequency and amplitude of the main signal component that it can be difficult to reliably detect the main signal component. Further, the frequency of the main signal component is determined by the rotational speed of the wheel. This increases the difficulty in detecting the main signal component since its frequency is initially unknown and is variable. As a result, conventional static filters cannot reliably extract the main signal component, especially when the vehicle is travelling over rough ground. A similar problem arises when other sensors, for example microelectromechanical systems (MEMS) sensors or accelerometers are used.
It would be desirable, therefore, to provide a filtering apparatus that is capable of reliably detecting electrical signals that emanate from apparatus on board rotating wheels.